Thirteen years after 9/11, an unsettling number of images from Ground Zero and its environs remain seared in our collective memory -- unsurprising, perhaps, given the scope and scale of the destruction. But the fact that the deadliest, most visually arresting attacks occurred in New York City also meant that many of the world's best photographers were, in effect, already on the scene when the terrorists struck. In hopes of lending coherence to our shared, turbulent recollections, LIFE.com presents the 25 most stirring, visceral photographs from that day, featuring pictures from the likes of James Nachtwey, Joe Raedle, Spencer Platt, Mario Tama, and other celebrated photojournalists (and one intrepid amateur). These are the pictures we remember: wrenching, indelible photographs that tell the tale of a still-resonant late summer day that changed everything.